This came up last night when trying to back up database files for apps on my Pre that don't do the auto back up. Fortunately, someone was kind enough to help me, so I'm posting this here in case any other Mac users come acorss the same issue.

1. Connect your Pre to your Mac using the micro USB cable
2. Put your Pre into USB Drive mode
3. Launch Terminal on your Mac
4. Type defaults write AppleShowAllFiles TRUE into Terminal, then hit Enter
5. Hold down the option-apple-Esc keys to bring up the Force Quit window. Select Finder, then click ReLaunch
6. All hidden files on the Mac and the Pre will now be visible, and file folders like .appstorage can be seen and dragged over to be backed up.
7. To hide the hidden files again, type defaults write AppleShowAllFiles FALSE into Terminal

I hope this helps someone!